I signed up for AdWords a long time ago and created a test campaign just to see what it looks like, but never completed the process, so I didn't have a payment method entered.
Fast forward till start of this year, I created an AdMob account to put ads in one iOS game I have, and apparently as soon as I entered my credit card info, the old AdWords campaign started running and was taking $200/day. I noticed after two days and immediately stopped it and contacted support. They quickly got back to me, a person called me and explained that the issue was an error from their side, and they refunded me the amount. It took three weeks for the refund to occur, and during that time the guy called me at least once per week to keep me in the loop. I was surprised with this after all I've heard about Google's support. But I guess that's just one data point in the pool.